Arsenic is a toxic heavy metal that is now subject to stringent environmental limits. Unfortunately, arsenic is found in many minerals, ore bodies, concentrates and other materials that also contain other valuable components. For example, enargite is a copper sulphide that is increasingly found in ores and concentrates that contains appreciable quantities of arsenic, for example, between 20 and 25% by weight, of arsenic. Speiss, which is formed during smelting of lead and other metals, can also contain appreciable quantities of arsenic.
It is frequently desirable to treat such solid materials to recover the valuable components. However, the treatment of such materials must take into account the arsenic content of those materials. Conventional treatment of such arsenic-containing materials by smelting can result in the emission of volatile arsenic, causing significant occupational health and safety issues, as well as causing concerns from an environmental viewpoint.
One method for treating such arsenic containing materials involves subjecting the material to a leaching step to selectively remove arsenic from the material. A solid/liquid separation step is then utilised to separate a pregnant liquor containing dissolved arsenic from the solid material. The solid material recovered from this leaching process has a reduced arsenic content. Desirably, other toxic heavy metals, such as antimony and bismuth, may also be removed from the solid material during the leaching process. Following this treatment, the materials having reduced arsenic content can then be treated to recover the other valuable components, for example, by pyrometallurgical or hydrometallurgical processes.
Existing processes for removing arsenic from sulphide containing materials typically involve leaching the materials with an alkaline solution that contains sodium sulphide (Na2S). This solution typically includes sodium hydroxide (NaOH) and sodium sulphide (Na2S). For example, in the treatment of enargite (a copper mineral generally accepted to contain Cu3AsS4), the following reaction is reported in literature:2Cu3AsS4(s)+3Na2S(aq)=3Cu2S(s)+2Na3AsS4(aq)  (1)
Any orpiment (As2S3, a common monoclinic arsenic sulfide mineral) that may be present reacts with the sodium sulphide according to the following equation reported in literature (2):3Na2S(aq)+As2S3=2Na3AsS3(aq)  (2)
The above equations show that the stoichiometric amount of Na2S required to fully remove all the arsenic present is 1.5 times the number of moles of arsenic present. In practice, commercial processes utilise an amount of Na2S that provides a mole ratio of sulphide to arsenic typically the range of 6 to 12. It has been found that arsenic removal is quite slow if an approximately stoichiometric amount of Na2S is used.
U.S. Pat. No. 3,911,078 describes a process for removing arsenic and antimony from copper sulphide containing materials. This patent describes the use of leaching solutions containing sodium hydroxide and sodium sulphide. The patent states that it is preferable in leaching to use about 2 to 3 times the stoichiometric quantity of sodium sulphide, or about 3 to 4.5 moles of Na2S for every mole of arsenic and every mole of antimony present. The patent further states that the presence of this excess Na2S assures essentially complete removal of the arsenic and antimony. Of course, the requirement to use in excess of Na2S incurs increased operational costs due to the requirement to purchase increased amounts of Na2S to feed to the leaching process.
As mentioned above, most alkaline sulphide leaching processes for removing arsenic from sulphide containing materials use a sulphide S2−/As molar ratio that is well above the stoichiometric requirement of S2−/As=1.5. Typically, a molar ratio of S2−/As of between 6 and 12 is used to achieve arsenic removal of over 90%. Generally, the processing conditions used in known alkaline sulphide leaching processes include temperatures in the range of 80 to 95° C., slurry densities from 10 to 50% w/w and a molar ratio of S2−/As of between 6 and 12.